A photocurrent flows through one end of a first pn junction with a photoelectric conversion function in an electrically floating state and is accumulated as charge for a certain period of time or accumulated charges are discharged by the photocurrent as will be described later. To detect a result of this as an electric signal, generally, the electric capacitance connected with said one end of the first pn junction is excessively charged or discharged if optical intensity is large relative to accumulation time or integral time. This makes the photocurrent flow forward in the first pn junction. As a result, the first pn junction is biased by a deep forward voltage and excess minority carriers are accumulated in the two semiconductor regions of opposite conductivity types forming the first pn junction. This causes a problem that response speed is degraded because of a delay due to so-called saturation time taken for switching the first pn junction to a reverse bias associated with the lifetime of minority carriers. This phenomenon is called a saturation effect.
Japanese Patent Publication No. S47-18561 discloses a technique in which the pn junction is connected in parallel with Schottky junction, in order to prevent the pn junction from being biased by a deep forward voltage. The pn junction can be prevented from being deeply biased because a forward voltage is smaller at the Shottky junction than at the pn junction so that most currents flow into the Shottky junction. However, reverse currents at the Shottky junction are several orders of magnitude larger than at the pn junction, which increases the total amount of dark currents. Because of this, this technique cannot be adopted for a high-sensitive photoelectric converter.
Herein, the first pn junction biased by a deep forward voltage refers to the forward voltage at the first pn junction when all the photocurrents flow therein as a forward current. If the amount of the photocurrents flowing as the forward current is reduced to one-tenth or less to decrease the saturation time to about one-tenth, the saturation problem is considered to be solved. In this case the forward voltage at the first pn junction is smaller than that in the deeply biased state by 2.3 kT/q (about 60 mV at room temperature). Herein, under the ambient condition in which the pn junction becomes biased by the deep forward voltage if left uncontrolled, the control over the pn junction to be maintained in zero bias or reverse bias state and to have the forward voltage smaller than this deep forward voltage by 2.3 kT/q is referred to as saturation control, where k is Bolzman constant, T is absolute temperature of the photoelectric converter, and q is elementary charge of electron.
If the first pn junction is formed of a first semiconductor region of a first conductivity type and a second semiconductor region of a second conductivity type opposite to the first conductivity type adjacent thereto, accumulated excess minority carriers are spread from the first pn junction within the diffusion length of minority carriers in both of the first and second semiconductor regions. The diffusion lengths differ depending on the type of carriers, electron or hole, or the electric characteristics of the semiconductor regions and the diffusion lengths between the first and second semiconductor regions are different.
Further, with a second pn junction having a photoelectric conversion function additionally provided in the diffusion length, currents flow in the second pn junction even if the second pn junction is not exposed to light, causing the photoelectric converter to malfunction. This leads to image blurs and equivalently degraded resolution in an imaging device comprising a photoelectric converter array in which photoelectric conversion elements as the pn junctions are arranged in the first semiconductor region.
Now, referring to FIG. 1, using a photodiode as an example of a photoelectric conversion element, how to convert optical information as optical intensity and wavelength components into electric information for output is described. FIG. 1 shows a photodiode 1000a as the first pn junction having an anode 1002a and a field effect transistor 3000a operating as a switch, in which the anode 1002a is connected with the source or drain of the field effect transistor 3000a. The field-effect transistor 3000a switches off the anode 1002a of the photodiode 1000a to place it in a floating state during accumulation time in order to temporarily accumulate photocurrents in the electric capacitance associated with the photoelectric conversion element (in this case, anode-cathode electric capacitance) and switches it on to output accumulated charges as current or charge.
First, the field effect transistor 3000a is switched on so that one end (anode 1002a in the drawing) of the photodiode 1000a is at a Vref potential and the anode-cathode electric capacitance of the photodiode 1000a is charged with a Vdd-Vref voltage. Vdd is a power supply voltage and Vref is a read reference voltage.
Next, the field effect transistor 3000a is switched off and the photoelectric conversion element is illuminated with light. Then, a photocurrent separately generated at the pn junction flows into the anode-cathode electric capacitance Canc from the anode 1002a of the photodiode 1000a and the capacitance charged with Vdd-Vref alone is discharged. Thereby, a cathode potential rises towards the power supply voltage Vdd. Thus, the anode-cathode electric capacitance is discharged by the photocurrent in reality. However, it may be expressed herein that a photocurrent is stored as a charge for convenience.
With a long switch-off time or a large photocurrent, the anode 1002a of the photodiode 1000a exceeds Vdd and reaches a forward potential. The forward potential continues to rise and reaches the maximum value when all the photocurrents flow through between the anode and cathode of the photodiode 1000a. This is referred to as “biased by a deep forward voltage”. In this state excess minority carriers are stored in the semiconductor regions of the photodiode 1000a, causing a delay in switching the photodiode 1000a to a reverse bias direction.